Spontaneous uterine rupture is a rare, but serious complication of placenta percreta. This case report is about a spontaneous uterine rupture at an unusual site due to placenta percreta in a 21-week twin pregnancy with previous cesarean section. A 30-year-old, G3, P2 woman was referred to our unit in the 21st week of a twin pregnancy with acute abdomen. An emergency laparotomy was performed with the diagnosis of uterine rupture and intra-abdominal hemorrhage. A significant hemoperitoneum was found, with both fetuses freely floating in the peritoneal cavity. A large transverse rupture at the posterior isthmus wall was detected. Subtotal hysterectomy with preservation of both ovaries was performed. Pathological investigation of the uterus revealed placenta percreta.